Method for producing oxide coated iron powder of controlled resistance for electrostatic copying systems

ABSTRACT

Oxide coated iron powder required as a carrier in electrostatic copying systems has a uniform oxide film, and has a resistance which can be selectively controlled between 102 and 1010 ohms, when produced by a five step program of: Step 1 - fluidizing iron powder in air while heating to a temperature between 500* and 750*F, to initiate an exothermic reaction; and thereafter to a higher peak temperature between 750* and 1600*F, desirably to about 950*F, principally by the heat of exothermically oxidizing iron, which can be supplemented by external heating. Step 2 - introducing an inert gas such as N2 into the air and maintaining the temperature of the iron powder essentially constant at its peak temperature. Step 3 - discontinuing the flow of air but maintaining fluidization by introducing a stream of inert gas such as N2, Ar or He while reducing the temperature to between 125* and 700*F. Step 4 - introducing air into the inert gas and cooling the fluidized iron powder to a temperature between 25* and 50*F below that at the end of step 3. The temperature at the beginning of this step determines the resistance of the product, the lower the temperature the lower the resistance, and vice versa. Step 5 - discontinuing the inert gas but maintaining fluidization in air while cooling the iron powder to a lower temperature suitable for removal.

United States Patent McCabe et al.

[ Oct. 23, 1973 METHOD FOR PRODUCING OXIDE COATED IRON POWDER OF CONTROLLED RESISTANCE FOR ELECTROSTATIC COPYING SYSTEMS Inventors: John M. McCabe; John S.

Perlowski; Ronald J. Schur, all of Rochester, N.Y.

[73] Assignee: Eastman Kodak Company,

Rochester, NY.

Filed: Dec. 27, 1971 Appl. N0.: 212,172

[52] US. Cl 148/635, 117/26, 117/31,

117/D1G. 6, 148/203, 423/634 Int. Cl. C23c 11/08 Field of Search 148/635, 20.3;

1 17/31, DIG. 6, 26; 423/634 [56] References Cited UNITED STATES PATENTS 4/1940 Jacobs 148/635 X 12/1944 Neighbors 148/635 X Primary Examiner-Edward G. Whitby Att0meyWilliam T. French et al.

[57] ABSTRACT Oxide coated iron powder required as a carrier in Alf? BEG/NS EXTERNAL HEAT/N6 STEPZ I l l l i electrostatic copying systems has a uniform oxide film, and has a resistance which can be selectively controlled between 10 and 10 ohms, when produced by a five step program of:

Step 1 fluidizing iron powder in air while heating to a temperature between 500 and 750F, to initiate an exothermic reaction; and thereafter to a higher peak temperature between 750 and 1600F, desirably to about 950F, principally by the heat of exothermically oxidizing iron, which can be supplemented by external heating.

Step 2 introducing an inert gas such as N into the air and maintaining the temperature of the iron powder essentially constant at its peak temperature. Step 3 discontinuing the flow of air but maintaining fluidization by introducing a stream of inert gas such as N Ar or He while reducing the temperature to between 125 and 700F.

Step 4 introducing air into the inert gas and cooling the fluidized iron powder to a temperature between 25 and 50F below that at the end of step 3. The temperature at the beginning of this step determines the resistance of the product, the lower the temperature the lower the resistance, and vice versa.

Step 5 discontinuing the inert gas but maintaining fluidization in air while cooling the iron powder to a lower temperature suitable for removal.

7 Claims, 3 Drawing Figures All? E(2550F be/aw D) STEP 3 TIME PAIENIEDHN 23 ma SHEET 10F 2 V QMR m MSQQMQ w Gum JOHN M. M CABE JOHN .S. PERLOWSK/ RONALD J SCHUR INVENTORS All? COOL/N6 AIR 6A3 HEATER EL ECTROME TER 25 JOHN M. M CASE JOHN S. PERLOWSK/ RONALD J. sci/ INVENTORS Byjgw 7,

A TTORNEY METHOD FOR PRODUCING OXIDE COATED IRON POWDER OF CONTROLLED RESISTANCE FOR ELECTROSTATIC COPYING SYSTEMS BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a novel method for producing oxide-coated iron (including alloy steel) powder of controlled resistance, suitable for use as a carrier of toner particles in an electrostatic copying system.

2. The Prior Art Iron powder has been used in the past as a carrier for toner particles to be used in electrostatic copying systems, such as the well known xerographic process. Sometimes such iron powder is coated with a polymeric material which improves the electrostatic copying action. In such systems it is important that the iron powder have a controlled resistance which may have different values for different specific operations. The carrier resistance is a primary factor in determining image development characteristics, and results ranging from full fringing to full solid-area development, and points between, can be obtained depending on the mass resistance of the iron particles. The higher the powder resistance, the more fringy is the image, and vice versa. Control to secure the resistance for any desired image characteristic has been difficult, if not impossible, in the past.

SUMMARY OF THE INVENTION In accordance with the present invention the iron powder required as a carrier in electrostatic copying systems has been greatly improved so as to have an extremely uniform oxide film on each particle, which is also uniform from particle to particle. Furthermore, the resistance of the oxide coated iron particles can be controlled to have selected specific values in the range between and 10 ohms, which is uniform throughout various batches of powder; and the oxide coated iron particles also have improved ability to be uniformly coated with a polymeric material when such a coating is desired. This improved iron powder is secured by oxidizing iron powder while it is continuously fluidized with gas in a reaction chamber A five step program is employed, with continuous uninterrupted fluidization, as follows: I

Step l fluidizing the mass in a st ream of air while heating to a temperature between 5 0()-750F., desirably to about 650F, to initiate an exothermic reaction therein; and thereafter to a higher peak temperature between 750 and 1600F, desirably to about 950F, principally by the heat of exothermically oxidizing iron, which can be supplemented by external heating.

Step 2 introducing an inert gas into the stream of air and maintaining the temperature of the iron powder essentially constant at its peak temperature. The time at this peak temperature determines the weight percent of oxygen pick up, but not the resistance.

Step 3 discontinuing the flow of air into the reaction chamber, but maintaining fluidization therein by introducing a stream of inert gas such as N Ar or He therein and reducing the temperature of the iron powder to a lower temperature between l 25 and 70W:

e.g. about 560F.

Step 4 introducing air into said stream of inert gas and cooling the fluidized iron powder to a temperature between z's' 'and 50F below that at the end of step 3, and thusly continuing incremental increases of air until the temperature of powder is below its reignition point. The inert gas acts to prevent reignition while the temperature is above the ignition temperature, for example above about 50O F f belowthat temperature air cali supplant the inert gas without danger of exothermic reaction.

Step 5 discontinuing the inert gas but maintaining fluidization by again introducing a stream of air into the reaction chamber and reducing the temperature of the iron powder to a lower temperature suitable for removing the final oxidized iron powder product from said reaction chamber.

In the program or cycle outlined above, the most critical point for assuring that the desired resistance will be obtained is at the-end of step 3 and the beginning of step 4. The resistance can be any selected v alue yvi thin ftlie range 10 toTO ohms depentfin g on the temperatureat that point. g

The fluidized oxidation process described above can be carried out by the apparatus described her ein, or in We reactor desEifiaTed iii UI'S'. Pat NT)? 3 mm granted Nov. 14, 1967 to John S. Perlowski (one of the present inventors) and William E. Sillick, entitled Process For Manufacturing Magnetic Oxide, and as signed to Eastman Kodak Company like the present application.

THE DRAWINGS THE PREFERRED EMBODIMENT T Tn' performingouTiidvel mammihtfi'arm eganaes EH sponge iron powder (-V'MSIT-FTKYITIESHUET Standard) are loaded into a fluidized bed reaction chamber 11 to provide a bed depth-to-width ratio greater than 1. The powder then isfluidized with air supplied at about 68 scf/min. by aconduit l3 and passing up through a perforate plate 15. The air may be at room temperature, or may be electrically or gas heated at?) to 30O4I) O F toexpedite theprocess.The reactor is externally heated by the surrounding electrical resistance heaters 19, or by radiant heaters. Temperatures are measured by a thermocouple 20.- The time in the various steps is not critical; but the longer the time at peak temperature in step 2, the greater the amount of oxidation occurring.

A program for producing iron powder having a resistance of T6 30 ohms will be described to exemplify the principles of the invention:

1 Step 1 The temperature in the reaction chamber rises gradually to 630F, atpoint A on the graph of FIG. 1 at which point an exothermic reaction of iron; with the oxygen in the air commences, and the rate of heating rapidly increases spontaneously. At 700F heating of the air at 17 is stopped. Time to point B can;

hours with unheated air.

short time in this instance to 530F at point E,

r2 2,,W sn.thsem rsw reaches 900F at point B external heating is discontinued and the exothermic reaction is controlled so as to hold the temperature a t 95 F: lF to accomplish the desired controlled oxidation. This is done by introducing an inert gas such as nitrogen into the air stream so that the air to nit ro g e nratio is approximately 1:3 and at the same time externally cooling the reactor by circulating air through an external cooling annulus 21. Time between aisaysd Q bsahq liminm a- Step 3 After a suitable length of time, such as about 33 minutes, the oxidizing reaction is stopped at point C by discontinuing the flow of air and fluidizing the bed of iron particles only with an inert gas such as unheated nitrogen supplied at 68 scf/min., but without any interruption in fluidization. The bed of iron particles is cooled down over about 20 minutes to about 560F at point D by the inert gas, and by continued circ ulat i on of cooling fluid outside the reaction chamber.

Step 4- At point D unheated air flowing at 25 scf/min. is again introduced into mixture with the nitrogen flowing at 68 scf/min. The temperature is reduced to about 30 below the temperature at point D over a Point D is extremely critical to the process because the temperature at which the oxidizing gas mixture is first applied determines the bulk resistance of the final oxidized iron particles. Temperatures in steps 1 and 2 are less critical, as long as oxidation occurs. The purpose of the added N is to prevent reignition of the iron powder at the higher temperatures at point D. At lower temperatp r e s such as l 50F, where reignition cannot occur, one can switch directly to air without addedN Step 5 The cycle is then completed by discontinuing nitrogen addition and cooling the mass, while fluidigedinunheated airat 63 s cf/min, down to room temperature or any other desired temperature, such as 150F or lower, at which it is feasible to remove the oxidized iron powder product from the reaction chamber.

When using a 560F temperature at point D and 530F at point E, the final product has a bulk resistamze of to lgflohms. At other temperatures for point D ranging from l25F to 700F, controlled bulk resistances in the range of 10 to 10'" ohms can be obtained; the lower the temperature at D the lower the bulkrcsistance.

For example, the following relationships have been found between temperature at point D and resistance:

Temperature F Resistance ohms 150 10 -40 500 10 S50 10'' 600 10" 700 l0"--l0 The process steps described above produce similar results as to powder resistance when applied to other types of iron powder, such as non porous atomized iron powder, for exarrrple the 70 mesh (US, Standard) spherical steel powder sold by Whittaker Corporation (Nuclear Metals Division), West Concord, Massachusetts.

Resistance of powder is measured by the following device and procedure:

I. A resistance cell 25 comprises a 10 inches long, narrow horizontal brass trough 27 having a lead connected to one side of an electrometer 29 set to read voltage on the 30 scale (General Radio Company, type [230A Electrometer). A pair of Teflon members 31,32

extend vertically upwardly on opposite sides of the trough and are coextensive therewith, with their top surfaces acting as horizontal tracks parallel to the trough.

2. A magnetic brush 33 comprises a shouldered han- 4 59a fllsatessyl sal ma 37 one inch in diameter encased within a cylindrical brass guard 39 so that the sides and end of the magnet are enclosed. The guard has a lead which is connected to a second side of the electrometer.

3. 15 grams of powder are placed in a round aluminum weighing dish 2% inchesin diameter, and the end of the magnetic brush is dipped into the center of the dish containing the powder until the end of the brush is in contact with the powder, but without compressing it, to attract a mass of powder.

4. Then the brush carrying the attracted powder sample on its end is removed and placed vertically in the resistance cell at one end thereof, with the brush's end surface spaced 5/32 inch from the brass trough but with the iron powder 41 in contact with the trough, and with the handle in contact with and riding on the tops of the Teflon tracks. The brush then is moved uniformly along the Teflon tracks (into the drawing paper) toward the other end of the cell to give the adherent powder a cylindrical shape and size which will be uniform each time a measurement is made.

5. When the brush has nearly reached the end of the cell its movement is stopped and, after a one minute wait, a circuit is completed through the brass guard, the adherent powder, and the brass trough, and the resistance on the ohmmeter of the electrometer is read.

6. The procedure is repeated and the two readings are averaged.

The five step oxidizing program described in detail above is particularly advantageous for producing oxidized core carrier particles, for electrostatic copying systems, having a selected specific resistance within the range of 10 to 10 ohms which has not been possible by other methods known to applicants at this time. Moreover, extremely good reproducibility has been obtained from batch to batch, when all batches are treated on the same program.

Other important advantages of the fluidization technique are as follows:

1. Fluidization is most effective in bringing a reactive atmosphere in innermost contact with the particulate materials. More conventional approaches such as tray or continuous belt roasting would yield a broader non uniform spectrum of results because top layers are exposed to more of the atmosphere.

2. Heat transfer rates are extremely good in fluidized beds. In addition to shortening process cycles, the high heat transfer provides very uniform temperatures, which are difficult to obtain in static beds of iron powder.

3. Mixing rates are high under fluidization which contributes to product uniformity.

4. Sintering together of the particles to form a cake is a serious problem during the oxidization of iron powder by some procedures. This problem is minimized in a fluidized bed.

5. The reaction is strongly exothermic, and from a safety viewpoint an inert gas can be quickly introduced into fluidized beds to stop the reaction.

6. Subsequent coating of the iron powder particles with a polymer is improved.

ing continuously fluidizing with gas a mass of iron powder in a reaction chamber while oxidizing said powder particles in a five step program consisting essentially of:

Step 1 fluidizing said mass in a stream of air while heating to a temperature (A) between j QQf apd 150T i l t at an. th nn onfleei a and then to a higher peak temperature (B) between 750 and lQQOF at least principally by the heat of exothermically oxidizing iron; i Step 2 introducing an inert gas into said stream of air and maintaining the temperature of siad iron powder essentially at said peak temperature B; Step 3 discontinuing the flow of air into said reaction chamber, but maintaining fluidization therein by introducing a stream of inert gas therein and reducing the temperature of said iron powder to a temperature D between 125 and 700l-T; Step 4 introducing air and cooling the fluidized iron powder to a temperature below temperature D under conditions precluding reignition of said iron powder;

6 Step 5 continuing flu'idization of said iron powder while reducing the temperature of said iron powder to a lower temperature suitable for removing the oxidized iron powder product from said reaction chamber.

2. A method in accordance with claim 1 wherein said inert gas in steps 2, 3 and 4 is nitrogen.

3. A method in accordance with claim 1 wherein the temperature at point D is about 700F for maximum powder resistance of between 10 and 10 ohms. 4. A method in accordance with claim 1. wherein the temperature at point D is about F for minimum pqwqe r staa a .10 9hms- 5. A method in accordance with claim 1 wherein, in step 4, said fluidized iron powder is cooled to a temperature between about 25 and about 50F below temperature D.

6. A method in accordance with claim 1 wherein the temperature D is above the ignition temperature of said iron powder, and wherein in step 4 said air is introduced into said stream of inert gas.

7. A method in accordance with claim 1 wherein the temperature D is below the ignition temperature of said iron powder, and wherein in step 4 said air supplants said inert gas, and the cooling of fluidized'iron powder in air proceeds through steps 4 and 5 to said lower temperature suitable for removing the final oxidized product from said reaction chamber. 

2. A method in accordance with claim 1 wherein said inert gas in steps 2, 3 and 4 is nitrogen.
 3. A method in accordance with claim 1 wherein the temperature at point D is about 700*F for maximum powder resistance of between 109 and 1010 ohms.
 4. A method in accordance with claim 1 wherein the temperature at point D is about 125*F for minimum powder resistance of 102 ohms.
 5. A method in accordance with claim 1 wherein, in step 4, said fluidized iron powder is cooled to a temperature between about 25* and about 50*F below temperature D.
 6. A method in accordance with claim 1 wherein the temperature D is above the ignition temperature of said iron powder, and wherein in step 4 said air is introduced into said stream of inert gas.
 7. A method in accordance with claim 1 wherein the temperature D is below the ignition temperature of said iron powder, and wherein in step 4 said air supplants said inert gas, and the cooling of fluidized iron powder in air proceeds through steps 4 and 5 to said lower temperature suitable for removing the final oxidized product from said reaction chamber. 